Want to get rich out of making video games? Don't worry about winning awards for your graphic artistry – hire some psychologists instead. They can tell you how the simplest of games can hijack our brain's evolved instincts to keep players hooked. Sally Adee and Douglas Heaven

Matching just one row of sweets triggers a cascade of further matches as more sweets pour down. Unexpected rewards like this excite us and the motivation systems in our brains encourage us to repeat what we were doing in the hope of getting another hit.

This ties into something called variable-ratio schedules of reinforcement – when you learn that performing a particular action can bring you a reward, but you don't know how much you have to do to get it. Tests on rats in the 1950s showed that if the amount of times the animals had to press a lever to get a reward varied, they would work the lever hundreds of times even after the rewards stopped coming. Sound familiar?

Video games often have several different variable-ratio schedules set up at once, with high scores, bonuses and level-up rewards all overlapping.